For example, Patent Documents 1 and 2 disclose using a friction roller-type speed reducer to reduce rotation of an output shaft of a small electric motor rotating at high speed and then to transmit the same to driving wheels such that efficiency of the electric motor as a driving source of an electric vehicle is improved to lengthen a mileage per one time charging. FIGS. 13 to 18 show a friction roller-type speed reducer disclosed in Patent Document 2.
A friction roller-type speed reducer 1 is configured to rotationally drive a sun roller 3 by an input shaft 2 to transmit rotation of the sun roller 3 to an annular roller 5 via a plurality of intermediate rollers 4, 4, and to take out rotation of the annular roller 5 from an output shaft 6. The respective intermediate rollers 4, 4 are configured to only rotate on rotation axes 7, 7 provided at respective center parts thereof and do not revolve around the sun roller 3. The sun roller 3 is configured by concentrically combining a pair of sun roller elements 8, 8 having the same shape, and a pair of loading cam devices 9, 9 are provided at positions at which the loading cam devices sandwich the sun roller elements 8, 8 from both axial sides. The respective parts are accommodated in a stepped cylindrical housing 10 of which a diameter of an axially intermediate part is larger and diameters of both end portions are smaller.
A base half part (a right half part in FIG. 13) of the input shaft 2 is rotatably supported to an inner side of an input-side small-diameter cylindrical part 11 of the housing 10 by an input-side ball bearing unit 12, and the output shaft 6 is rotatably supported to an inner side of an output-side small-diameter cylindrical part 13 by an output-side ball bearing unit 14. The input shaft 2 and the output shaft 6 are concentrically arranged, and a tip portion of the input shaft 2 is supported to an inner side of a circular concave portion 15 formed at a center portion of a base end surface of the output shaft 6 by a radial rolling bearing 16. A base end portion of the output shaft 6 is coupled to the annular roller 5 by a coupling part 17 having an L-shaped section.
The sun roller elements 8, 8 are arranged concentrically with the input shaft 2 around a tip half part of the input shaft 2 so as to be rotatable relative to the input shaft 2 with a gap being interposed between tip surfaces (facing surfaces) of the sun roller elements. A pair of circular plate-shaped cam plates 18, 18 configuring the loading cam devices 9, 9 are externally fitted and fixed at two positions of an intermediate portion and a tip portion of the input shaft 2, at which the cam plates 18, 18 sandwich the sun roller elements 8, 8 from both axial sides, and is configured to rotate synchronously with the input shaft 2. On the base end surface of each of the sun roller elements 8, 8 and one surface of each of the cam plates 18, 18, which surfaces face each other, driven-side cam surfaces 19, 19 and driving-side cam surfaces 20, 20 are provided at a plurality of positions in the circumferential direction, respectively. Balls (rolling elements) 21, 21 are respectively interposed between the respective cam surfaces 19, 20, so that the loading cam devices 9, 9 are configured. An axial depth of each of the cam surfaces 19, 20 gradually changes in the circumferential direction. That is, the axial depth is deepest at a center portion in the circumferential direction and becomes shallower towards both end portions.
When torque is input to the input shaft 2, a surface pressure of each traction part, which is a rolling contact part between the circumferential surfaces of the respective rollers 3 to 5, is increased, as follows. First, at a state where the torque has not been input to the input shaft 2, the respective balls 21, 21 configuring the loading cam devices 9, 9 exist at bottoms of the respective cam surfaces 19, 20 or at sides close to the bottoms, as shown in FIG. 14A. At this state, the thickness of the loading cam devices 9, 9 is small and an interval between the sun roller elements 8, 8 is wide. Also, each of the intermediate rollers 4, 4 is not pressed outward in a radial direction of the sun roller 3 and the annular roller 5, and even when it is pressed by an elastic force of a preload spring, for example, the pressing force is small.
From the above state, when the torque is input to the input shaft 2 (the friction roller-type speed reducer 1 is activated), the axial thicknesses of the loading cam devices 9, 9 increase based on engagement between the respective balls 21, 21 and the respective cam surfaces 19, 20, as shown in FIG. 14B. Then, the sun roller elements 8, 8 contact inner sides of the respective intermediate rollers 4 with respect to the radial direction of the friction roller-type speed reducer 1, thereby pressing the respective intermediate rollers 4 outward in the radial direction. As a result, the surface pressure of each traction part increases, so that it is possible to transmit power from the sun roller 3 to the annular roller 5 without causing excessive sliding to each traction part. The loading cam devices 9, 9 to be incorporated into the friction roller-type speed reducer 1 have springs provided between the sun roller elements 8 and the cam plates 18 configuring the respective devices and configured to apply elastic forces of relatively displacing both the members 8, 18 in the circumferential direction. Both the members 8, 18 are relatively displaced in the circumferential direction based on the elastic forces of the springs, so that the respective balls 21, 21 override the shallow sides of the respective cam surfaces 19, 20. As a result, it is possible to apply the preload to each traction part by the loading cam devices 9, 9.
During the operation of the friction roller-type speed reducer 1, the respective intermediate rollers 4, 4 rotate about the respective rotation axes 7, 7 and are simultaneously displaced in the radial direction of the friction roller-type speed reducer 1 as the transmission torque is varied. The reason is that the larger the pressing force generated by the loading cam devices 9, 9, the forces of pressing the respective intermediate rollers 4, 4 towards an inner peripheral surface of the annular roller 5 by the loading cam device 9, 9 increase. In order to smoothly perform the rotation and radial displacement of the respective intermediate rollers 4, 4, in the friction roller-type speed reducer 1, the respective intermediate rollers 4, 4 are provided in an annular space 22 between the inner peripheral surface of the annular roller 5 and an outer peripheral surface of the sun roller 3 by a following structure, for example. In order to support the respective intermediate rollers 4, 4, a support frame 25 as shown in FIGS. 15 and 16 is supported and fixed to an inner surface of an end plate 24 which closes one axial side of a large-diameter cylindrical part 23 of the housing 10. The support frame 25 has a structure like a carrier configuring a planetary gear mechanism, and has a pair of circular-ring shaped rim parts 26a, 26b arranged concentrically with each other and coupled and fixed each other at a plurality of positions equally spaced in a circumferential direction by stays 27, 27. The rim parts 26a are screwed to an inner surface of the end plate 24, so that the support frame 25 is supported and fixed to an inner side of the large-diameter cylindrical part 23 concentrically with the sun roller 3.
The respective intermediate rollers 4, 4 are rotatably supported to tip portions of swing frames 28, 28. Each of the swing frames 28, 28 has a pair of support plate parts 29, 29 parallel with each other and coupled at base end edges thereof by a base part 30 to have a U shape, as seen from a radial direction. End portions of the rotation axes 7, 7 of the respective intermediate rollers 4, 4 are respectively rotatably supported to tip portions of the support plate parts 29, 29 of the respective swing frames 28, 28 by ball bearings 31, 31. Also, swing shafts 32, 32 protruding from both side surfaces of a base end portion of each of the swing frames 28, 28 are inserted into support holes 33, 33 formed at matching portions of the rim parts 26a, 26b without any rattling.
The respective swing shafts 32, 32 and the respective rotation axes 7, 7 are parallel with each other and phases in the circumferential direction of the support frame 25 are largely offset. Specifically, in order to make the offset in the circumferential direction of the respective swing shafts 32, 32 and the respective rotation axes 7, 7 as large as possible, a direction of a virtual line connecting each of the swing shafts 32, 32 and each of the rotation axes 7, 7 is made close to a direction of a tangential line to a virtual arc having a center at a center of the support frame 25.
An outer peripheral surface of each of the intermediate rollers 4, 4 has a shape in which an axially intermediate portion is configured as a simple cylindrical surface and both portions thereof are configured as inclined surfaces having a partially conical convex surface shape inclined at the same angle and in the same direction as outer peripheral surfaces of the sun roller elements 8, 8.
The outer peripheral surfaces of the base end portions of the sun roller elements 8, 8 are provided with collar parts 34, 34 having an outward flange shape, respectively. That is, parts of the outer peripheral surfaces of the sun roller elements 8, 8, which are to rolling-contact the outer peripheral surfaces of the respective intermediate rollers 4, 4, are configured as inclined surfaces which are inclined in a direction along which an outer diameter thereof gradually decreases towards a tip surface. The collar parts 34, 34 protrude radially outward over the entire circumference from base end portions of the inclined surfaces. The base end surfaces of the sun roller elements 8, 8, including the collar parts 34, 34, are formed with the respective driven-side cam surfaces 19, 19.
The conventional friction roller-type speed reducer 1 configured as described above is configured to transmit the power from the input shaft 2 to the output shaft 6 while reducing the speed and increasing the torque, as follows. That is, when the input shaft 2 is rotationally driven by an electric motor, the cam plates 18, 18 externally fitted to the input shaft 2 are rotated, so that the sun roller elements 8, 8 are pressed in a direction of coming close to each other and are rotated at the same speed and in the same direction as the input shaft 2 based on the engagement between the respective balls 21, 21 and the respective cam surfaces 19, 20. Then, the rotation of the sun roller 3 configured by the sun roller elements 8, 8 is transmitted to the annular roller 5 via the respective intermediate rollers 4, 4 and is taken out from the output shaft 6. The surface pressure of each traction part is secured to some extent from a time when the friction roller-type speed reducer 1 is activated, by a cam part pressing force generated based on the elastic forces of relatively displacing both the members 8, 18 in opposite directions, which forces are provided by the springs provided between both the members 8, 18. Therefore, from the time of the activation, the power transmission starts at each traction part without causing the excessive slip.
When the torque applied to the input shaft 2 increases, overriding amounts of the respective balls 21, 21, which configure the loading cam devices 9, 9, on the respective cam surfaces 19, 20 increase and the axial thicknesses of the loading cam devices 9, 9 further increase. As a result, the surface pressure of each traction part is further increased and the large torque is transmitted at each traction part without causing the excessive slip. The surface pressure of each traction part is a value obtained by multiplying an appropriate safety factor by a proper value corresponding to the torque to be transmitted between the input shaft 2 and the output shaft 6, specifically a requisite minimum value, and is automatically adjusted.
Also, based on the swing displacement of the respective swing frames 28, 28, the respective intermediate rollers 4, 4 are smoothly displaced in the radially outward direction of the sun roller 3 and the annular roller 5.
The friction roller-type speed reducer 1 as described above has room for improvement, from a standpoint of improving transmission efficiency. That is, during the operation of the friction roller-type speed reducer 1, while the input shaft 2 is rotating, the respective balls 21, 21 configuring the loading cam devices 9, 9 rotate (revolves) between the sun roller elements 8, 8 and the cam plates 18, 18. The respective balls 21, 21 are strongly pressed to portions (portions surrounded by a dashed-dotted line α in FIG. 19) near the radially outer sides of the respective cam surfaces 19, 20 by the centrifugal force applied to the respective balls 21, 21 based on the rotation. Therefore, the axial pressing force to be generated by the loading cam devices 9, 9 has a summed magnitude of a force, which is to be generated by increasing the overriding amounts of the respective balls 21, 21 on the respective cam surfaces 19, 20 and an axial component force Fx of a force based on the centrifugal force. The force based on the centrifugal force is determined by a rotation speed of the input shaft 2 and a contact angle between the respective balls 21, 21 and the respective cam surfaces 19, 20. In the conventional structure as described above, it is difficult to regulate the contact angle with high precision. For this reason, the axial pressing force generated by the loading cam devices 9, 9 becomes excessively large, so that the surface pressure of each traction part excessively increases. As a result, the transmission efficiency of the friction roller-type speed reducer 1 may be lowered.
Regarding the above problems, it is considered to prevent the axial force to be generated by the loading cam device from excessively increasing by providing a circular ring-shaped retainer, which is configured to bear the centrifugal force to be applied to the respective balls configuring the loading cam device and to regulate the radial positions of the respective balls to appropriate states, between the cam plate and the sun roller element. However, even when the retainer is provided, following problems may be caused. That is, when the interval between the cam plate and the sun roller element increases as the loading cam device operates, the retainer may come down between the cam plate and the sun roller element or axially rattle (an axial position of the retainer is offset from a center position of the gap between the cam plate and the sun roller element). Thereby, a center axis of rotation of each ball may be inclined from a normal state. Patent Documents 3 to 5 disclose a technique of forming projections on both axial side surfaces of a retainer and preventing the retainer from coming down by the projections. However, in the structure disclosed in each of Patent Documents 3 to 5, the respective projections are formed at portions of the axial side surfaces of the retainer, at which phases in the circumferential direction are the same as pockets. For this reason, the stress is more likely to be concentrated on the portions at which the respective pockets are formed, so that the strength and stiffness of the corresponding portions may be lowered.
Also, the friction roller-type speed reducer 1 as described above has room for improvement from standpoints of securing the durability and improving the transmission efficiency. That is, a limit value (limit traction coefficient μmax) of a traction coefficient with which power can be transmitted without causing harmful slip referred to as gross slip at each traction part is changed under influences of conditions except for the torque to be transmitted between the input shaft 2 and the output shaft 6. For example, as shown with a solid line a in FIG. 10 showing a third aspect of a friction roller-type speed reducer to be described later, as a peripheral speed v of a traction part (rotation speed of outer peripheral surfaces of the sun roller 3 and the respective intermediate rollers 4, 4) increases, the limit traction coefficient μmax decreases (necessary pressing force increases). In contrast, according to the conventional friction roller-type speed reducer, as shown with a broken line b in FIG. 10, the traction coefficient of each traction part is constant, irrespective of the peripheral speed v. For this reason, when the peripheral speed v is low, the pressing force of each traction part excessively increases (an excessive pressing state is formed), the durability and the transmission efficiency may be lowered. In contrast, if the pressing force is properly made when the peripheral speed v is low, the gross slip is likely to occur at each traction part when the peripheral speed v is high.